Varactors are commonly used in various applications that require capacitors with variable capacitance values. Varactors are capacitors whole capacitance values change with the voltages applied on the capacitors. For example, in Voltage Controlled Oscillators (VCOs), varactors are widely used.
Varactors are commonly formed using Metal-Oxide-Semiconductor (MOS) devices. In a typical varactor, a gate of a MOS device is used as one capacitor plate of the varactor, and the source and drain of the MOS device are interconnected to form another capacitor plate of the varactor. The gate dielectric acts as the capacitor insulator. In the VCOs, low sensitivity and low KVCO (the gains of the VCOs) are typically needed, wherein the sensitivity is the ratio of the capacitance change to the change of the bias voltage applied on the gate.